1. Technical Field
Processes and apparatuses for removing organic sulfur compounds (e.g., thiophenes) from liquid hydrocarbon streams are disclosed. After subjecting a liquid hydrocarbon stream to oxidation conditions, thereby oxidizing at least a portion of the organic sulfur compounds (e.g., oxidizing thiophenes to sulfones), the oxidized organic sulfur compounds are reacted with caustic (e.g., sodium hydroxide, potassium hydroxide, etc.) to produce sodium sulfite organic compounds.
2. Description of the Related Art
The presence of sulfur in petroleum fuels is a major environmental problem. Indeed, the sulfur is converted through combustion into various sulfur oxides that can be transformed into acids, thus contributing to the formation of acid rain SOx emissions also reduce the efficiency of catalytic converters in automobiles. Furthermore, sulfur compounds are thought to ultimately increase the particulate content of combustion products.
Because of these issues, reduction of the sulfur content in hydrocarbon streams has become a major objective of environmental legislation worldwide. For instance, Canada, Japan, and the European Commission have all recently adopted a 0.05 wt % limit on sulfur in diesel fuels. In Sweden and the United States (in particular, California), the total sulfur content of gas oils is limited to 0.005 wt %. This limitation could eventually become the standard in the countries belonging to the OECD. In France, the total sulfur content in gasoline is currently limited to 0.05 wt %, but this limit is anticipated to be lowered to 0.005 wt % for all of Europe.
Refiners typically use catalytic hydrodesulfurizing (“HDS”, a.k.a. “hydrotreating”) methods to lower the sulfur content of hydrocarbon fuels. In HDS, a hydrocarbon stream that is derived from a petroleum distillation is treated in a reactor that operates at temperatures ranging between 575 and 750° F. (˜300 to ˜400° C.), a hydrogen pressure that ranges between 430 to 14,500 psi (3000 to 10,000 kPa or 30 to 100 bats) and hourly space velocities that range between 0.5 and 4 h−1. Thiophenes in the feed react with the hydrogen when in contact with a catalyst arranged in a fixed bed that comprises metal sulfides from groups VI and VIII (e.g., cobalt and molybdenum sulfides or nickel and molybdenum sulfides) supported on alumina. Because of the operating conditions and the use of hydrogen, these methods can be costly both in capital investment and operating costs.
Unfortunately, HDS or hydrotreating often fails to provide a treated product in compliance with the current strict sulfur level targets. This is due to the presence of sterically hindered sulfur compounds such as unsubstituted and substituted thiophenes that act as refractory compounds in hydrotreating environments. For example, it is particularly difficult to eliminate traces of sulfur using such catalytic processes when the sulfur is contained in molecules such as dibenzothiophene with alkyl substituents in position 4, or 4 and 6. Attempts to completely convert these species, which are more prevalent in heavier stocks such as diesel fuel and fuel oil, have resulted in increased equipment costs, more frequent catalyst replacements, degradation of product quality due to side reactions, and continued inability to comply with sulfur requirements.
One attempt at solving the thiophene problem discussed above includes selectively desulfurizing thiophenes contained in the hydrocarbon stream by oxidizing the thiophenes into a sulfone in the presence of an oxidizing agent, followed separating the sulfone compounds from the rest of the hydrocarbon stream. Oxidation has been found to be beneficial because oxidized sulfur compounds can be removed using a variety of separation processes that rely on the altered chemical properties such as the solubility, volatility, and reactivity of the sulfone compounds.
Oxidation techniques for converting thiophenes to sulfones vary and include: contact with a mixture hydrogen peroxide and a carboxylic acid to produce sulfones, which are then degraded by thermal treatment to volatile sulfur compounds; the oxidation of thiophene and thiophene derivatives in the presence of a dilute acid, with the sulfones being extracted using a caustic solution; a combination of the oxidation and thermal treatment steps with hydrodesulfurization; a two-step oxidation and extraction method extracting with a paraffinic hydrocarbon comprising a 3-6 carbon alkane; and various catalytic oxidation processes Techniques for the removal of the sulfones or “sulfone oil” include extraction, distillation, and adsorption. Another strategy involves decomposing the sulfones compounds catalytically thereby bypassing the separation process altogether.
An intrinsic problem of oxidative desulfurization lies in the disposal of the sulfones. If the sulfones are hydrotreated, they may be converted back to the original dibenzothiophene compounds thereby regenerating the original problem. Therefore, oxidative desulfurization is preferably used as a polishing step after hydrodesulfurization (HDS) or hydrotreatment. The feed sulfur content is likely to be in the range of 100 to 300 ppmw sulfur. Sulfur, on average, comprises about 15 wt % of substituted and unsubstituted dibenzothiophene molecules. Therefore, from about 0.06 to about 0.20 wt % of the oil is removed as sulfone extract. For typical refinery producing 40,000 barrels per day of diesel, approximately 7,000 to 20,000 pounds per day of sulfone oil will be generated, which is too much to dispose conventionally as a waste product. Further, the disposal of sulfone oil also wastes valuable hydrocarbons, which could theoretically be recycled if an efficient process were available.
Therefore, there is a need for a process for regenerating or recycling sulfone oil thereby avoiding disposal problems associated with the sulfone oil waste stream.